Toothpaste of selected calcined alumina particles

ABSTRACT

There is disclosed a toothpaste comprising as an abrasive an alumina in the form of particles having d 10  below 3.5 μm, d 50  below 1.0 μm and a specific surface area below 6 m 2 /g.

International standard ISO11609 of 1995 contains the following definitions:

Dentifrice: any substance or combination of substances specially prepared for the public for cleaning the accessible surfaces of teeth.

Toothpaste: any semi-solid dentifrice preparation presented in the form of a paste, cream or gel.

This invention is concerned with toothpaste.

Toothpaste invariably contains an abrasive powder for cleaning purposes. The cleaning and abrasive properties of the toothpaste depend on the concentration of the powder, on the Moh's hardness of the abrasive, and on the shape and particle size and size distribution of the powder. The cleaning effect of toothpaste concerns its effectiveness in removing adventitious stains and other deposits from the surface of teeth and other parts of oral cavity. The abrasive effect or abrasivity concerns undesired removal of surface components of teeth including enamel and dentine, and undesired damage in the oral cavity. A toothpaste having a high cleaning effect generally has a rather high abrasivity, and vice versa. ISO11609 provides methods for testing abrasivity and sets limits on dentifrices marketed in Europe. An earlier British standard BS5136 of 1981 also sets limits on abrasivity by comparison with a standard reference toothpaste, which is also used as a reference in ISO11609, and which has the formulation:

Precipitated calcium carbonate 40% w/w.

Glycol 25% w/w

Sodium carboxymethylcellulose 1.40% w/w

Dodecyl sodium sulphate 1.00% w/w

Sodium silicate (80° TW of approximately pH 7) 0.05% w/w

Saccharin sodium 0.15% w/w

Formalin (40% (m/m) formaldehyde) 0.10 w/w

Peppermint flavouring 0.80% w/w

Water 33.05% w/w

Abrasive powders used or contemplated for use in toothpaste formulations include silicas, including gels and precipitates, sodium bicarbonate, calcium and magnesium carbonates, calcium phosphates, alumina and hydrates thereof, aluminosilicates, aluminium and magnesium silicates, and thermosetting urea-formaldehyde and other plastics materials. There is a need in the industry for an abrasive for inclusion in toothpaste formulations, which provides good cleaning properties, in particular stain removal, while showing relatively low abrasivity. In particular, there is a need for an abrasive which can be incorporated in a concentration large enough to provide an excellent cleaning effect in a toothpaste with low abrasivity which nevertheless complies with the abrasivity requirements of the above-stated standard specifications. This invention addresses that need.

U.S. Pat. No. 3,957,968 teaches the use of flat flakes of α-aluminium oxide in toothpaste. The flakes have a median particle size of 2 to 7 μm. U.S. Pat. No. 4,060,599 discloses the use of a finer aluminium oxide (median 1 to 2 μm) and specifically uses Reynolds RC152DBM which has a median size of around 1.7 μm.

U.S. Pat. No. 4,632,826 teaches the use of a weakly calcined alumina polishing agent. This polishing agent consists of 10% to 50% by weight γ aluminium oxide and 50% to 90% by weight α aluminium oxide.

GB-A-2155333 teaches the use of calcium hydrogenphosphate anhydride and an aluminium oxide having an average particle size of 0.5 μm to 10 μm. The alumina has a high α-content, as measured by X-ray diffraction.

WO-A-95/33441 teaches the use of cationically charged colloids of a metal compound. The colloid has a particle size of 0.001 μm to 0.2 μm.

GB-A-2037162 and GB-A-2009596 disclose the use of hydrated aluminas in dentifrice products.

The phrase “alumina” is sometimes loosely used to cover a number of aluminium oxide, oxide hydroxide and trihydroxide compounds. The correct designations and some crystalline phases are shown below:

Accepted Mineral Crystallographic and Name Chemical Composition Chemical Designation Gibbsite Aluminium trihydroxide γ-Al(OH)₃ Bayerite Aluminium trihydroxide α-Al(OH)₃ Nordstrandite Aluminium trihydroxide Al(OH)₃ Boehmite Aluminium oxide hydroxide γ-AlOOH Diaspore Aluminium oxide hydroxide α-AlOOH Corundum Aluminium oxide α-Al₂O₃

This invention is concerned with alumina that is essentially Corundum. Corundum is produced by calcination of aluminium trihydroxides and oxide hydroxides. Depending on the form of aluminium trihydroxide and oxide hydroxide starting material, a number of forms of alumina (often described as activated aluminas) are produced before corundum is formed. Another form of alumina chemical is aluminium hydroxide gel, which is often formed by the neutralisation of an aluminium salt solution.

The invention provides a toothpaste comprising as an abrasive an alumina in the form of particles having d₁₀ below 3.5 μm, d₅₀ below 1.0 μm, and specific surface area below 6 m²/g.

According to a further aspect of the present invention, there is provided the use of alumina as an abrasive in toothpaste, wherein the alumina is in the form of particles having d₁₀ below 3.5 μm, d₅₀ below 1.0 μm, and a specific surface area below 6 m²/g.

According to a further aspect of the present invention, there is provided the use of alumina as a whitening agent in toothpaste, wherein the alumina is in the form of particles having d₁₀ below 3.5 μm, d₅₀ below 1.0 μm, and a specific surface area below 6 m²/g. In this way the alumina may serve the purpose of replacing titania or the like in conventional products, thus resulting in a cost saving.

The surface area is measured by the following method. A sample of alumina of sufficient weight to give an estimated surface area of about 0.5 to 25 m² is degassed in a Micrometrics Desorb 2300B at about 150° C. until a stable reading is obtained. The sample is then transferred to a Micrometrics Flowsorb II 2300, cooled and immersed in a mixture of 30% N and 70% He gas. The total amount of N absorbed is measured from the change in thermal conductivity of the gas mixture preferably during desorption as the temperature is raised again to room temperature. Surface area per gram is then calculated from the total gas absorbed and the weight of the sample.

The particle size is measured as follows. Particle size distribution of a sample of alumina is measured in a Sedigraph 5100 instrument supplied by Micrometrics Products.

The abrasive is preferably an anhydrous alumina, generally a calcined alumina or alternatively a tabular or fused alumina. Calcination is effected at a temperature of at least 900° C. Higher calcination temperatures result in harder products. The alumina used in this invention is preferably fairly hard, such as may be obtained by calcination at above 1000° C. The alumina in this invention is an α-alumina. Preferably, the α content is greater than 90%, preferably greater than 93%, even more preferably greater than 95% as measured by X-ray diffraction. In a preferred embodiment, the γ-content is less than 1%.

The abrasive is used in the form of particles having d₁₀ below 3.5 μm, preferably below 2.5 μm. (d₁₀, d₅₀ and d₉₀ are used in conventional manner to indicate that 10, 50 or 90 wt % of the product has a particle size above the stated value). Preferably, the alumina abrasive is a sub-micron product, having d₅₀ of 0.1-1.0 μm. Preferably the alumina abrasive has a relatively narrow particle size distribution, e.g. with d₁₀ being no greater than four times d₅₀.

The alumina abrasive has a specific surface area below 6 m²g, preferably in the range of 4.5-5.0 m²/g. The specific surface area is related to the aforesaid parameters of hardness (harder products have lower specific surface areas) and particle size (more finely divided products have larger specific surface areas).

Alumina products of the kind described are readily available commercially, for they are produced in substantial quantities mainly for use in refractories and ceramics. Grinding may conveniently be effected by fluid energy or vibratory milling (micronising) or preferably by ball milling.

When an abrasive powder has excellent stain removal properties in one toothpaste, it is generally the case that it will be found to have excellent stain removal properties in others. Although the abrasivity of a toothpaste does depend to a significant extent on the whole formulation, and not merely on the nature and concentration of the abrasive present in it, nevertheless an abrasive which shows high or low abrasivity in one toothpaste formulation may generally be expected to show correspondingly high or low abrasivity in others.

The alumina abrasive is preferably present in the toothpaste at a concentration of 1-15% w/w, preferably higher than 2%, even more preferably higher than 3%, e.g. 3-12% w/w. Toothpastes include a wide variety of components in a wide variety of concentrations. The alumina should be compatible with other ingredients. The following list is intended to be exemplary rather than definitive or restrictive.

Toothpastes are generally water-based.

A binder or thickener is generally present. Examples of suitable materials are carboxyvinyl polymers, carrageenan, hydroxyethyl cellulose and water soluble salts of cellulose ethers such as sodium carboxymethylcellulose and sodium carboxymethyl hydroxyethyl cellulose. Natural gums and colloidal silica or silicate materials can also be used. Binders or thickeners are generally present in an amount from about 0.15 to about 5.0% w/w of the total composition.

A humectant is also generally used to keep the formulation from hardening on exposure to air. Examples of suitable humectants are glycerine, sorbitol, xylitol, polyethylene glycols and propylene glycol. Humectants are generally present in an amount from about 10% to about 70% w/w of the weight of the composition.

One or more particulate materials, regarded as abrasives or abrasive polishers or fillers, are also present. The alumina abrasive described above is one such, but others may also be present as noted above. Examples are:

Silicas, including gels and precipitates, sodium bicarbonate, calcium and magnesium carbonates, dicalcium phosphate dihydrate, alumina and hydrates thereof, aluminosilicates, aluminium and magnesium silicates, and thermosetting urea-formaldehyde and other plastics materials. It is preferred that the alumina is not present in combination with calcium hydrogenphosphate anhydride.

Abrasives are generally present at a level of about 10% to about 70%, preferably from about 15% to about 25%, w/w of the formulation.

A source of fluoride ion is preferably provided. Regulatory authorities in various countries may stipulate a maximum and/or a minimum total fluoride ion concentration.

Other components which may also be included in toothpaste formulations in accordance with conventional practice, include sweeteners, flavours, colours, peroxides or other bleaching agents, anti-calculus agents, anti-plaque agents, anti-bacterial agents, preservatives and effervescence generators.

Toothpaste may be manufactured by conventional techniques. The alumina abrasives with which this invention is concerned are rather easy to incorporate for, unlike some other abrasive powders, they are generally not prone to lumping.

Here is an example of a toothpaste formulation:

Dicalcium phosphate 40% w/w

Alumina 10%

Sorbitol 25%

Polyethylene glycol 2%

Carboxymethylcellulose 1.1%

Sodium Saccharin 0.2%

Sodium Lauryl Sulphate 1.5%

Benzoic Acid 0.15%

Sodium Benzoate 0.2%

Water 19.5%

This formulation based on dicalcium phosphate polishing agent/filler, has been used to test the properties of various alumina abrasives. In the experimental section below, a different formulation based on a silica polishing agent/filler was used. Although the results obtained with the two formulations were different in absolute terms, they were generally the same in relative terms, i.e. any particular alumina abrasive generally had the same or a similar effect on the properties of both formulations. This justifies the assertion that an abrasive showing valuable properties in one toothpaste may reasonably be expected to show corresponding valuable properties in others.

The alumina abrasives used in the experimental section below are either commercial products or development products (designated BAX) available from Alcan Chemicals Limited, and are here identified by means of their trade names. This invention is based on the discovery that the development product BAX842 has unexpectedly and outstandingly good properties.

EXAMPLE 1

In this example, a silica-based toothpaste was used. The formulation was as shown in Table 1.

TABLE 1 Silica-based toothpaste formulation Ingredient % w/w Polyethylene Glycol 3.0 Sodium carboxymethyl cellulose 0.6 Deionised Water 7   Sodium Fluoride  0.243 10% Sorbitol  56.107 Sodium Saccharin 0.3 Titanium Dioxide 0.3 Abrasive Silica 20.5  Flavour  0.75 Sodium Lauryl Sulphate 1.2 Alumina 10.0  Total 100   

The alumina products used in this example are given in Table 2.

TABLE 2 Mineralogy/ SSA Sedigraph Product Milling route m²/g d₁₀ d₅₀ d₉₀ X Alumina/micronised 1.3 5.19 3.11 1.4  RA207LS Alumina/ball milled 8.2 1.01 1.4  0.18 BAX888 Alumina/ball milled 6.8 1.41 0.48 0.27 BAX842 Alumina/ball milled 4.8 2.35 0.62 0.36 BAX904 Alumina/ball milled 6.1 2.07 0.51 0.25 PA2 (1) Alumina/ball milled 6.7 8.24 1.18 0.47 PA2 (2) Alumina/micronised 4.4 6.83 0.99 0.51 MA250 Alumina/micronised 4.6 11.05  1.29 0.49 BAX 985A Alumina/ball milled 5.9 3.23 0.53 0.28 BAX 985B Alumina/ball milled 6.6 2.47 0.49 0.25

For stain removal, samples were subjected to an in-vitro test developed and performed by the Health Science Research Centre at Indiana University-Purdue University. This test involves staining teeth using a broth containing instant coffee, instant tea, gastric mucin3 and a micrococcus leteus culture. The colour of the stained teeth is measured using a Minolta colourmeter. The teeth are then brushed with the toothpaste and the colour measured. A measurement of stain removal is then calculated.

The enamel abrasivity (Designated REA) was measured by Missouri Analytical Laboratories, using a method described by R J Grabenstetter et al. (J D Res. Vol. 37, November-December 1958, No. 6, pages 1060-8).

The abrasivity and stain removal results are shown in Table 3.

TABLE 3 Abrasivity and stain removal results Enamel Abrasivity Stain Removal REA Value % X 149 55 RA207LS  45 42 BAX888  53 42 BAX842  46 50 BAX904  58 40 PA2 (1)  70 43 PA2 (2)  80 37 MA250 (3) 110 54 BAX 985A  86 50 BAX 985B  77 42

Discussion

The comparative performance of the aluminas in terms of abrasivity (REA value) and stain removal is shown in FIG. 1. BAX 842 is outstanding.

The relationship between particle size d₁₀ and enamel abrasivity is shown in FIG. 2. For the ball milled products, it can be seen in FIG. 2 that there is a linear relationship (R²=0.96, equation y=18.985x+25.109) between REA value and d₁₀ with REA falling as the d₁₀ decreases. BAX842 does not fit this linear relationship as the abrasivity is much less than would be expected for its d₁₀, i.e. for a d₁₀ value of 2.35 μm, the linear regression equation calculates an enamel abrasivity of 69 versus 46 obtained with BAX842. There is no relationship between enamel abrasivity and d₁₀ for the micronised aluminas. The relationship between stain removal and d₁₀ for the ball milled and micronised aluminas shows significant scatter in performance with no linear relationship. 

What is claimed is:
 1. A toothpaste comprising as an abrasive in a formulation of the toothpaste an alumina in the form of particles having d₁₀ below 3.5 μm, d₅₀ below 1.0 μm, and a specific surface area below 6 m²/g, wherein the alumina is at least 90% α-alumina calcined at a temperature of at least 900° C.
 2. The toothpaste of claim 1 wherein d₁₀ is below 2.5 μm.
 3. The toothpaste of claim 1 wherein the specific surface area is 4.5-5.0 m²/g.
 4. The toothpaste of claim 1, wherein the particles have d₅₀ of 0.1-1.0 μm.
 5. The toothpaste of claim 4, wherein the ratio of d₁₀ to d₅₀ is not more than 4.0.
 6. The toothpaste or claim 1, wherein the abrasive is present in a proportion of 1-15% w/w.
 7. The toothpaste of claim 1, wherein the alumina is ball-milled.
 8. The toothpaste of claim 1, wherein the alumina has an α content of greater than 90%.
 9. The toothpaste of claim 8, wherein the alumina has an α content of greater than 93%.
 10. An oral cleansing method comprising cleaning parts of an oral cavity with a toothpaste containing alumina as an abrasive, wherein the alumina is in the form of particles having d₁₀ below 3.5 μm, d₅₀ below 1.0 μm, and a specific surface area below 6 m²/g, and wherein the alumina is at least 90% α-alumina calcined at a temperature of at least 900° C., such that the toothpaste is effective in removing deposits from dental surfaces without undesired removal of dental surface components.
 11. An oral cleansing method comprising cleaning parts of an oral cavity with a toothpaste containing alumina as a whitening agent, wherein the alumina is in the form of particles having d₁₀ below 3.5 μm, d₅₀ below 1.0 μm, and a specific surface area below 6 m²/g, and wherein the alumina is at least 90% α-alumina calcined at a temperature of at least 900° C., such that the toothpaste is effective in removing deposits from dental surfaces without undesired removal of dental surface components. 